To date quorum sensing systems have been found in gram negative bacteria that engage in a symbiotic or pathogenic relationship with a higher eukaryotic host, e.g., plants, animals and humans. Three open reading frames, designated cerR, cerA, and cerI, are presumed to encode regulatory and structural proteins necessary for maintaining a quorum sensing system in Rhodobacter sphaeroides 2.4.1. This challenges the current view that R. sphaeroides 2.4.1 exists solely as a free-living bacterium. The proposed research is designed to elucidate the quorum sensing system in R. sphaeroides 2.4.1, and should indicate the type of interaction it may have with a higher eukaryote. The transcriptional organization and expression of the cer genes will be defined. The role of cerA, located within the cer transcription unit, is currently unknown. Through mutagenesis, a phenotype may be identified which should lead to its function. Reporter gene based transposon mutagenesis should establish the genes and operons that are regulated by the Cer proteins. Preliminary evidence suggests that multiple quorum sensing systems exist in R. sphaeroides 2.4.1, an approach will be used to verify the data. Completion of this project should help expand the concept of free-living prokaryotic-higher eukaryotic interactions.